ALEXANDRIA, Egypt  Thousands of Islamists clashed with their opponents Friday in Egypt’s second largest city, Alexandria, on the eve of the second leg of voting on the country’s contentious constitution that has deeply polarized the nation.

The two sides hurled rocks and stones at each other in the Mediterranean port city, prompting police to fire tear gas to separate them.

Volleys of tear gas containers fell into the sea as security forces cordoned off the crowds to prevent further clashes between the Muslim Brotherhood members and ultraconservative Salafis on one side, and groups of young protesters on the other.

It was not immediately clear who started the fight, which added to the already tense political crisis over the draft charter.

The Islamists had called for a big rally Friday outside the Qaed Ibrahim main mosque in the heart of Alexandria. About 20 political parties had issued a joint statement, saying they would not hold a rival rally in the city to avoid clashes.

Security forces cordoned off streets leading to the mosque as throngs of mostly long-bearded Salafi Islamists gathered for what they called “the million-man rally to defend clerics and mosques.” Islamists chanted “God is Great,” and warned opponents, “with blood and soul, we redeem Islam.”

The rally was called in response to last week’s violence, when a well-known Alexandria preacher and ultraconservative Salafi cleric, Sheik Ahmed el-Mahalawi, was trapped inside a mosque for 12 hours while his supporters battled rock-throwing opponents outside with swords and firebombs.